(1) Field of the Invention
The present invention relates to a silicon nitride-silicon carbide composite sintered material mainly containing silicon nitride and silicon carbide and also relates to a manufacturing method of the material. Particularly, this invention relates to a sintered material which is applicable to high-temperature structural materials and excellent in room-temperature strength, high-temperature strength and high-temperature creep properties, and also relates to a manufacturing method of the material.
(2) Description of Prior Art
Silicon nitride sintered materials have been well known as engineering ceramics excellent in strength, hardness, thermal stability and chemical stability, and application of such materials to structural materials particularly useful for heat engines is now advancing.
Such a silicon nitride sintered material can be obtained by admixing a sintering assistant agent, such as oxides comprising the elements of Group IIIa in the Periodic Table and the like components, to silicon nitride powder, molding the admixed material, and calcining the molded body at 1500.degree. C. to 2000.degree. C. in a non-oxidative atmosphere. However, the silicon nitride sintered material obtained in such a manner, though being excellent in almost all of the properties to be required, has poor strength at high temperature. To solve the problem on deterioration of the strength at high temperature, many kinds of counterplans by improving of the sintering assistant agent or by changing the calcining atmospheres or calcining patterns have been tried so far. However, satisfactory results can not be obtained yet.
On the other hand, silicon carbide sintered materials, though being inferior to the silicon nitride sintered materials in general strength, show little deterioration of the strength at high temperature.
Therefore, in view of such properties of these two kinds of sintered materials, a composite sintered material comprising adding silicon carbide to silicon nitride and calcining the obtained admixture is now proposed. Incidentally, because of deterioration of sintering ability in the composite sintered body to be caused by addition of silicon carbide, oxides of rare earth elements, such as Y.sub.2 O.sub.3 or the like component, and Al.sub.2 O.sub.3 or the like material are generally added to increase the density of the whole system of the composite material.
According to the above-mentioned prior art, it becomes possible to suppress the deterioration of strength at high temperature by adding silicon carbide to silicon nitride as compared with the case of silicon nitride sintered materials. However, from minute investigation on properties at high temperature of such a composite sintered material, we the inventors found that deterioration of strength or poor creep properties can be seen when the sintered material is left for a long time in a high temperature atmosphere under load. Therefore, by the poor creep properties, it becomes difficult to carry out long-time operation of, for example, a turbine rotor to which the sintered material is applied. Accordingly, such a material is not yet sufficient to be used in practice.
However, the creep properties, though being thought to be attributable to respective granular states in the sintered material, are not grasped completely. Therefore, the improvement of properties of the sintered materials by adding silicon carbide to silicon nitride can be further advanced if the mechanism of the creep properties can be well recognized.